Gray Walkers: A short story of the Geriatric Apocalypse

Imagine a zombie apocalypse. Now forget everything you know. This is an apocalypse of a different sort.

Billy is a normal teenager who's grandmother is suffering from Alzheimer's disease. One horrible day his world takes a turn for the worse. People are turning into aged, withered husks. Even children now appear to be octogenarians.
When his family succumbs to this geriatric plague, Billy must race to reach his girlfriend before they both become old before their time.

This short story is a warped take on the zombie apocalypse theme. You'll never look at grandma the same again.